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Bonding to Existing Concrete

posted by Bob Monday, November 22, 2010 at 12:00 PM

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.

.

454 USER COMMENTS

Joe, Sakrete Sand Mix with Sakrete Bonder and Fortifier as an admixture will provide the adhesion and depth up to 2” that is needed for this system. Prep the surface by removing any bond breakers such as paint, stain, dirt, dust, sealer, oil, etc. A French drain may be another alternative to think about.
- Chris Technical Services
Tuesday, April 28, 2015 at 10:46 AM

Kelly, the type of repair will determine which method of bonding is most effective. For example an addition to a driveway would require a Mechanical bond, whereas a patch 2” deep a chemical bond is recommended.
- Chris Technical Services
Tuesday, April 28, 2015 at 10:35 AM

Mike, the paint must be removed to ensure a quality bond. Sakrete Flo-Coat can be used to resurface areas from ½” down to a feathered edge. No bonding agent is necessary when using Flo-Coat it is already highly polymer modified for superior adhesion.
- Chris Technical Services
Tuesday, April 28, 2015 at 10:16 AM

My asphalt driveway is 200ft and downhill. It connects to a concrete pad that serves at the "turnaround" area that we back our cars out of the garage onto. When it rains, the rain flows down the driveway and onto the concrete, which has a slight pitch to the right. The 2' wide grass strip on the right edge of the concrete pad collects all of the rain water, staying wet all the time. I plan to put some berms on the asphalt to divert some of the water from coming down the hill. Still, I need to do something with the concrete. I'd like to put a small curb..couple of inches high x by 4" wide on top of the existing concrete and along that right edge. Theoretically, the water would then travel along that concrete curb to the low corner, where I have a drain that spills into a ditch a few feet to the right of this concrete pad (other side of grass area). This drain is downhill of the grass strip, so it will be allowed to dry thoroughly on hot days. The curb doesn't need to be high, but it needs to be wide enough to provide horizontal force in case a car tire backs into it while leaving the garage. I do not want to drill the existing slab and put vertical rebar or dowels in; I need the curb to hold itself. I think by making it only a couple of inches high, the tires would roll over it instead of against it. What product do you recommend for this? Will it eventually dry/stain to look like the existing slab? Thanks. Sorry for the length; tried to create mental picture.
- Joe
Sunday, April 26, 2015 at 7:54 PM

Which method is more effective for bonding new concrete to old mechanically or chemically.
- kelly
Saturday, April 25, 2015 at 7:38 PM

I have a area about 22'by 6'. Fully exposed to the weather I live in the north east. I have 2 areas that have water damage from water getting under concrete. Area is also painted. How do i repair. I have removed areas that are damaged from a skim coat to about 1/2" in depth or so. I understand the paint is a problem? Do i have to remove all the paint or is there another way to do this. HELP!
- Mike
Friday, April 24, 2015 at 7:08 PM

Roger, with the use of rebar and Sakrete High Strength Concrete Mix you should be able to repair this area without any trouble.
- Chris Technical Services
Monday, April 20, 2015 at 1:11 PM

Hi, Thank you for the informative article of pouring concrete over concrete, I fully intend to do this asap. I have an existing concrete slab whose house floated down the river 30 years ago. The slab remains in good shape, and I want to rebuild on it. Because the space is 30x20 I am thinking to use the Sakrete Bonder/Fortifier with the Sand Mix. Is that what you would recommend? Thanks again Kimberly info@spit-n-whittle.com
- Kimberly
Monday, April 20, 2015 at 12:11 PM

Dave, unfortunately this is not an easy quick fix. We suggest using Sakrete Sand Mix with Sakrete Bonder and Fortifier as an admixture. Sand Mix is used for repairs from 2” down to ½”. To blend the areas into the existing concrete use Sakrete Top ‘n Bond. Top ‘n Bond is used from ½” down to a feathered edge. Top ‘n Bond is polymer modified so it does NOT need any additional additives for adhesion. If you have any other questions please call us at 1-866-SAKRETE.
- Chris Technical Services
Monday, April 20, 2015 at 11:11 AM

Hello- I have an out building that large gravel/rock has been removed basically 144 sq. ft. area with a depth of approx. 6 inches. Without the expense of a concrete truck charging phenomenal prices what are your recommendations, thank you.
- Roger Buggle
Thursday, April 16, 2015 at 12:31 PM

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